The earth is missing at least 20 meters of sea level rise. The Antarctic could be the time bomb

About 115,000 years ago homo sapiens They still lived in groups of hunter-gatherers, mostly restricted to Africa. We have still shared the globe with the Neanderthals, though it is not clear that we have yet met them.

And though these different hominids did not know it, the earth was at the end of a great warm period. It was one that came pretty close to our current climate, but with a big discrepancy – the seas were 6 to 9 meters higher at that time.

During this ancient time, sometimes referred to as Eemian, the oceans were about as warm as they are today.

And last month, there was fascinating new research suggesting that northern hemisphere glaciers have already declined as much as in Eemian, driven by dramatic warming in the Arctic regions.

The findings emerged when a research team on Baffin Island in northeastern Canada examined the remnants of ancient plants that had appeared beneath the rapidly retreating mountain glaciers.

And they found out that the plants were very old and probably had grown last 115,000 years ago.

This is the last time that the areas were actually covered in ice, the scientists believe.

"It's very hard to find another explanation, except that at least in the one area we work in … the last century is as warm as any century in the last 115,000 years," said geologist Gifford Miller University of Colorado in Boulder, who led the research on Baffin Island.

But if Miller is right, there is a big problem. We have geological records of the sea level of the Eemian. The oceans were 6 to 9 meters higher according to the scientists.

There may have been some additional water from Greenland, whose ice currently has a potential sea-level rise of over 6 meters. But it could not only have been Greenland, because the entire ice sheet did not melt at that time.

Therefore, the researchers also suspect a collapse of the most vulnerable part of the Antarctic, the West Antarctic ice sheet. This region could easily provide another sea level rise of 3 meters or more.

"There is no way to reach tens of meters of sea-level rise without reaching tens of meters of sea-level rise in Antarctica," said Rob DeConto, an Antarctic expert at the University of Massachusetts.

Try to understand how the Antarctic will fall

Scientists are now debating intensively which processes could have had an effect at that time – and how fast they will play again. After all, the West Antarctic has shown again that it is retreating.

Some researchers, including DeConto, believe that they have come up with a key process – the so-called collapse of ice cliffs – that in a hurry can trigger a significant sea level rise from the West Antarctic.

However, they are being challenged by another group whose members suspect that the changes in the past have been slow – and will be again.

To understand the dispute, you should consider the vulnerable environment of the West Antarctic itself.

It's essentially a huge block of ice, mostly in very cold water. Its glaciers are facing the ocean in all directions, and towards the middle of the ice plate the seabed drops rapidly, even if the surface of the ice plate itself becomes much thicker, about two miles in total.

Up to one and a half miles of this ice rests below sea level, but there is still much ice over that.

So, as the Gateway glaciers move backwards – especially a glacier called Thwaites, by far the largest of them – the sea would have quick access to much thicker ice.

The idea is that during the Eemian, this whole area was not an ice block at all, but an unnamed sea. Somehow the ocean came in, overturning the outer defenses of the glaciers, gradually flooding the entire West Antarctic and melting it on course.

DeConto and his colleague David Pollard built a model that looked at the Eemian and another ancient warmth called Pliocene to understand how this could happen.

In particular, they included two processes that can be used to remove glaciers. One, referred to as "sea ice instability," describes a situation where a partially submerged glacier becomes deeper and thicker the farther you move toward the center.

In this configuration, warm water can cause a glacier to move backwards and forwards, exposing ever thicker ice to the ocean – and thicker ice will flow out faster.

The loss is nourished by itself.

In West Antarctica, instability of the marine ice sheet is probably already underway, but in the model this was not enough. DeConto and Pollard added another process currently underway in Greenland on a large glacier called Jakobshavn.

Jakobshavn moves backwards down a subterranean hillside as it is feared that the much larger Thwaites will drift. But Jakobshavn does something else as well. It constantly breaks off thick pieces at the front, almost like a loaf of bread dropping slice after slice.

This is because Jakobshavn no longer has an ice board, a floating extension that used to grow out of the glacier over the ocean and stabilize it. The shelf collapsed when Greenland had warmed over the last two decades.

As a result, Jakobshavn now presents a steep vertical front to the sea. Most of the glacial ice is under water, but more than 100 meters in length – and that's the problem for DeConto and Pollard. That's too much to be sustainable.

Ice is not steel. It breaks. And breaks. And breaks.

This extra process, referred to as the "Sea Ice Cliff Breakdown", will cause a complete disaster if you use it on Thwaites. If one day Thwaites loses his own ice shelf and shows a vertical front to the sea, you will have ice cliffs hundreds of feet above the water surface.

DeConto and Pollard say such cliffs would constantly fall into the sea. And when they added that calculation, not only was the sea-level rise in Eemia restored, but so was the forecast of how much ice Antarctica could give in this century – more than three feet.

As there are other drivers of sea-level rise, such as Greenland, this meant that we could see a total of six feet in this century, which is about twice the previous projection. And in the next century, ice loss would get worse.

"We pointed out that if the species of calves we see today in Greenland burn in Antarctica in analogous environments, then the Antarctic will have much thicker ice, which means that a larger ice sheet will really make the consequences enormous could be for the sea level rise, "said DeConto.

Moreover, the process is essential to understanding the past – and how we could replicate it.

"We can not restore a six meter rise in sea level early in the Eemian without considering a brittle fracture in the Ice Shield model," said DeConto.

A massive debate on sea ice cliffs

Tamsin Edwards is not convinced. A glaciologist at Kings College London, she is the lead author of a study published Wednesday in London with a number of other antarctic experts nature(the same magazine in which DeConto and Pollard were published in 2016), which denies her model in detail.

Using a statistical technique to examine the results, Edwards and her co-workers find that falling the ice rocks is not necessarily necessary to reproduce past warm periods.

They also provide opportunities for rising lower Antarctic sea levels in this century. If they are right, the worst case is back to about 40 centimeters, or just over a foot lower than three to four feet.

"Things may not be as horrible as the last study that was predicted," Edwards said. "But they are still bad."

It is a new science, and without further modeling, it is unclear how ice cliffs will eventually affect sea level rise.

But what happened in Eemian? Edwards believes it took a long time to lose West Antarctica. That it was not fast. After all, the entire geological period was thousands of years long.

"We are impatient, humans, and the ice cover has not responded for a decade, they are slow beasts," she said.

DeConto says he learned something from the critique.

"The Edwards study illustrates the need for more detailed statistics than we originally used for our 2016 model edition, but the models are evolving rapidly and have changed significantly since 2016," he said in a written statement.

But he does not retire on sea ice rocks. The new criticism, DeConto said, implies that "these processes are not important for future sea-level rise, and I think that's some kind of dangerous message."

He certainly has his allies. Richard Alley, a well-known glaciologist at Penn State University, who has published with DeConto and Pollard, wrote in an e-mail that "The Cliff Retreat is not some weird and unexpected physical process, it is happening in some places now, in of the past, and is expected there, where sufficiently high temperatures occur in the sea or in the air around the ice flowing into the sea.

The Eemianer – but worse?

It is an important thing to keep in mind – the Eemian occurred without people emitting much greenhouse gases.

Atmospheric carbon dioxide was much lower than it is today. The event was instead driven by changes in Earth orbit around the sun, causing more sunlight to fall on the northern hemisphere.

The big difference this time is that people are heating up things much faster than they've been supposed to in the geological past.

And that makes a big difference, said Ted Scambos, an Antarctic explorer who directs the US side of an international multi-million dollar mission to study Thwaite's Glacier, and a senior researcher at Colorado's National Snow and Ice Data Center is.

"The current pace of climate change is very fast," said Scambos, and the warming could cause glaciers to behave differently than in the past.

Accordingly, Scambos says he sees the current debate as fruitful – "it is the discussion that needs to happen" – but does not diminish his concern about the fate of Thwaites Glacier if he withdraws far enough.

"There is no model that says the glacier does not accelerate when it gets into that situation," said Scambos. "It just has to be."

People were nowhere near the Antarctic in the Eemian Sea – and in modern times we have never seen a glacier as big as the retreat of Thwaites. Maybe something will happen for which we have no precedents or predictions.

For example, just last week, scientists reported a large cavity opening under one part of the glacier – something they said models would not have predicted.

There is a lot of effort now to find out what might happen – before it actually happens. It will help determine if people who are now organized and industrialized and master fossil fuels are ready to advance a reiteration of our own geological history.